The Public Switched Telephone Network (PSTN) has become ubiquitous for wireline telecommunications of voice and/or data. FIG. 1 is a high-level block diagram of a telecommunications system that uses digital transmission networks to provide a connection from a beginning point of a circuit, often called the A location, and a terminating point of the circuit, often called the Z location. As shown in FIG. 1 the telecommunications system 100 includes first Customer Premise Equipment (CPE) 110 at a first customer location A that is connected to a first Customer Premise Equipment (CO) 120, for example using a copper transmission medium 112. In one example, the circuit between the first customer location A and the first central office 120 may be a T1 circuit that contains 24 channels. At the first central office 120, signals from the first customer location A is multiplexed with many other signals and transported to a second central office 130 over copper and/or fiber transmission media 122. At the second central office 130, the signals are demuliplexed and sent to a second customer location Z having second CPE 140, for example using a T1 fiber connection 132. The overall operation of the telecommunications system 100 is well known to those having skill in the art and need not be described further herein.
FIG. 2 is a block diagram of pieces of electronic equipment at or connected to a central office, such as the central offices 120 or 130 of FIG. 1. Referring to FIG. 2, a conventional equipment layout of a central office includes low speed equipment 210, a cross-connect panel or cross-connect frame 220, a multiplexer (MUX) 230, a fiber distributing frame 240 and optical fibers 250. Multiple pieces of each type typically may be used.
Low speed equipment 210 is a general term that can apply to many different types of equipment. Conventionally, the term may refer to any digital transmission equipment that feeds DS1 and/or DS3-rate signals over copper wire to the low speed side of an optical fiber multiplexer. Low speed equipment can include Subscriber Loop Carrier (SLC) systems, channel banks, asynchronous multiplexers and/or other pieces of central office equipment.
Cross-connect panels 220, often referred to as DSX panels, provide a connection point between two types of digital transmission equipment. Each cross-connect panel 220 is dedicated to a piece of digital transmission equipment. The panel allows a connection between the equipment to which it is wired and other digital transmission equipment. Cross-connect panels 220 also provide jack access to the transmission paths between those two pieces of equipment. The jacks on cross-connect panels 220 provide test points for monitoring, testing and/or troubleshooting digital transmission systems. In one example, the low speed side of a multiplexer 230 that transmits and receives DS1 and/or DS3 signals, is wired to a cross-connect panel 220. From the cross-connect panel 220, cross-connect wiring is connected to another piece of transmission equipment, such as low speed equipment 210. Thus, the transmission path is mechanically cross-connected between the two pieces of digital transmission equipment. Cross-connect panels 220 that connect equipment at the DS1 level often are referred to as DSX-1 panels. Cross-connect panels 220 that connect equipment at the DS3 level often are referred to as DSX-3 panels.
Continuing with the description of FIG. 2, the multiplexer 230 combines multiple signal streams into one or more signal streams at a higher bit rate. By combining several signal streams into one or more signal streams at a higher bit rate, multiplexers can send more data over less fiber or copper wire. For example, a fiber optic multiplexer, often referred to as a Synchronous Optical NETwork (SONET) multiplexer, operating at the OC-3 rate, can multiplex 3 DS3 signals or 84 DS1 signals into an OC-3 signal. During the multiplexing process, the SONET multiplexer receives DS1, DS3, and/or other types of digital signals on the low speed side of the multiplexer. Circuit packs on the low speed side of the SONET multiplexer take the digital signals and reformat them into synchronous optical signals. Circuit packs on the high speed side of the SONET multiplexer take the reformatted SONET signals from the low speed circuit packs and multiplex them into a higher bit rate and then convert them into an optical signal. The high speed circuit packs then send optical signals over fiber jumpers to the fiber distribution frame (FDF) 240 and on to another locations via outside plant fiber 250. The FDF 240 is a shelf within a central office that terminates outside plant fiber. The FDF 240 provides a convenient interface between the outside plant fiber 250 and fiber optic equipment in the central office.
The design, operation and functionality of equipment located at a central office are well known to those having skill in the art and need not be described further herein. Moreover, as used herein, the term central office refers to any location in a digital transmission system that employs a cross-connect frame. As such, the term central office also encompasses Remote Terminals (RT), which are other locations in a digital transmission system between a central office and a customer.
FIG. 3 is a schematic diagram of a cross-connect frame, also referred to as a cross-connect panel or a DSX panel, such as the cross-connect frame 220 of FIG. 2. As was described above, a cross-connect frame provides a connection point between two types of digital transmission equipment and provides jack access to the transmission paths between those two pieces of equipment. FIG. 4 is a more detailed wiring diagram of DSX-1 cross-connect panels. As shown in FIGS. 3 and 4, the cross-connect panel is permanently wired to central office equipment. All of the jacks on an entire cross-connect frame can be permanently wired to just one piece of equipment as shown in FIGS. 3 and 4, or respective sets of jacks on a cross-connect frame can be wired to respective different pieces of equipment.
For example, in FIG. 4, all of the jacks on a first DSX-1 cross-connect panel 410 are dedicated to the DS1 ports on the low speed side of a SONET MUX 420. On a second DSX-1 panel 440, only four jacks are dedicated to a SLC 450, because the SLC 450 transmits and receives a maximum of only four DS1s. Other jacks on the second DSX-1 440 may be dedicated to other DS1 transmission equipment, such as other SLCs and/or channel banks.
The permanent wiring from the cross-connect panel to the associated central office equipment is usually from pins on the rear of the cross-connect panel to pins on the equipment. The cross-connections which allow for circuit connections between two pieces of equipment are also generally made by wiring between pins on the rear side or below the jack panel. The jack panel may be used for jack access for testing and/or monitoring of the cross-connected transmission paths.
Each circuit on the cross-connect panel may include a monitor (MON), an OUT jack and an IN jack. The IN jack connects to the transmission path going into the equipment. The OUT jack connects to the transmission path coming out of the equipment. Plugging into the IN and OUT jacks disconnects the cross-connection, so that traffic is interrupted for the entire length of time that the plug is inserted. Therefore, tests performed by plugging in to the IN and OUT jacks on a DSX are intrusive tests. The MON jack is connected to the OUT jack and is isolated by resistors. This allows for nonintrusive “in service” tests of the OUT signal without interfering with transmission.
One type of intrusive test is a loop test, also referred to as a loop back test. In a loop test, a patch cord is used to loop the OUT jack from a DSX back into the IN jack below it. When a loop is set at one location and a test set is connected to a DSX at another location, a signal can be sent from the test set to the other location and back to itself through the loop back. The signal levels, bit error rates and/or other test measurements of the loop back signal can help to determine if there is trouble in the transmission path between the test set and the loop back at the other location.
Accordingly, loop connections are often made in a cross-connect panel to perform various testing, maintenance and/or other operations. For example, in a designed carrier circuit, such as a switched base T1 service to the CPE, there generally needs to be a continuous path through the entire circuit. If continuity is broken, an immediate alarm may be activated. In many cases, the cause of the alarm is that the customer has disconnected the CPE due to a disconnect order that has not yet been processed in the switch. A hardware loop may be made to deactivate the alarm. Also, for new service, the central office equipment is turned up first and then the outside equipment is installed. In order to prevent an alarm from triggering in both of the above scenarios, a hardware loop is placed on the DSX frame to provide continuity of the signal.
In another example of a loop connection, service orders often are issued for disconnects, but it was not the intention of the customer to disconnect service, because the customer changes its mind after the initial disconnect request. In order to confirm that the circuit is to be disconnected, the central office technician would either place a hard loop in the DSX frame or back out the circuit card in the DSX frame to see if the customer calls in a trouble for an out of service state. Thus, a central office technician can verify a disconnect order without having to break down the entire circuit. However, this procedure is service affecting and can take some time to restore service.
As a final example of a loop connection, when a subscriber calls in trouble to the carrier, a testing technician will perform a series of tests to attempt to isolate the trouble. However, if a central office technician placed a hard loop in error, or followed company procedure for disconnecting service, the testing technician may see no trouble whatsoever and may have to make an uneducated guess to the cause of the trouble (customer premise, outside plant or central office) due to the testing technician's inability to see the trouble on the circuit because the hard loop is in place. Accordingly, although desirable and necessary in many circumstances, the set up, maintenance and removal of loops may consume time and/or cause problems in the telecommunications system.